Polymeric propellant and liner composition using a trimer acid

ABSTRACT

SOLID PROPELLANT MATRICES AND LINER COMPOSITIONS CONTAINING TRIMER ACID AS AN ADDITIVE TO IMPROVE THE MECHANICAL PROPERTIES THEREOF.

United States Patent 01 ice 3,649,389 Patented Mar. 14, 1972 3,649,389 POLYMERIC PROPELLANT AND LINER COMPOSI- TION USING A TRIMER ACID Benny G. Barron, Lake Jackson, Tex., assignor to North American Rockwell Corporation No Drawing. Filed Dec. 9, 1966, Ser. No. 601,296 Int. Cl. C06d /06; F42b 13/00 US. Cl. 14919 7 Claims ABSTRACT OF THE DISCLOSURE Solid propellant matrices and liner compositions containing trimer acid as anadditive to improve the mechanical properties thereof.

This invention relates to a method and resulting com.- position for improving the binder matrix in solid propellant grains and rocket motor liners for utilization with reinforced solid propellants.

, In copending application Ser. No. 144,265, filed Oct. 10, 1961, there is disclosed a solid propellant rocket motor grain which has circumferentially wrapped aluminum wire therein which serves both as the fuel for the composition and more importantly, to provide a high strength grain, having excellent mechanical properties. Additionally, other forms of solid propellant grains having metal reinforcements therein have been disclosed and are known in the art. In order to further improve the properties of such metal reinforced solid propellant grains, attention is'directed to improving the strength of the matrix propellant used in such grains. Thus, it has been found, through this invention, that a high strength matrix binder in the reinforced grain improves the characteristics of the propellant in several major respects. As will be shown, the improved propellant binder of this invention has higher longitudinal shear strength. There is improved wire to matrix bonding properties. Additionally, there is improved wire support that has been observed with less tendency for the wire to cut through the propellant matrix. There is increased compression and bending characteristics. Further, there is bonding between the case liner and the main propellant when the herein invention isutilized.

Generally, in the reinforced propellant grains referred to above the uncured liner is applied directly to the grain and cured before the case is wound over the grain. The liner bonds directly to the grain. The case bonds directly to the liner. No other adhesive is used. The liner, which is'conventionally used in many solid propellant motors where case bonding between the grain and the case is desired, serves to insulate the case from heat generated by the propellant, serves as a high strength material to inhibit cracks in the grain, and absorbs stresses along the case wall. The herein invention will improve the properties of the liner in a similar manner to improving the properties of the propellant as previously described. Thus, by using the additive of this invention, a high elongation tensile strength and improved modulus are obtained in the liner. Furthermore, the liner of this invention utilizing the additive disclosed will have good .bond strength to the matrix propellants and to the case wall which often will be fiber glass. Additionally, the liner has good insulating properties to prevent burnthrough and is of sufficient hardness to support fiber glass rovings during the case winding operation, when the case is wound directly on the grain.

The above improvements in both propellant and liner materials are achieved by the herein invention, which utilizes trimer acid such as Emery trimer acid made by Emery Industries, Inc., as an additive when mixing the liner and propellant matrix formulations. The invention is peculiarly applicable to propellants and liners having as a binder material polymers that are terminated with carboxy groups. Examples of these are carboxy terminated linear polybutadiene, polybutadiene acrylic acid polymers, and the terpolymer of polybutadiene, acrylonitrile and acrylic acid. The polymer binder material used in both the solid propellant compositions and in the liner utilizes such carboxy terminated polymers.

The trimer acid is an exceedingly complex structure. It may be approximated by the reaction of 9,12-linoleic acid with 9,11-lino1eic acid in accordance with the Diels- Alder Reaction to obtain a linoleic acid dimer. The dimer is then further reacted with 9,1l-linoleic acid through either of the vinyl linkages present to give the trimer acid. The material is well known in the art and is usually noted as Emery trimer acid denoting the major supplier. In the propellant matrix compositions, the trimer acid comprises from .8 to 1.10 weight percent of the total matrix composition. In the liner, a much greater amount of the trimer acid is utilized. In liners, the trimer acid comprises from 10 to 20 weight percent of the total composition.

In order to obtain a solid coherent mass, the binders in solid propellants and in the liner compositions must be cross linked. This is accomplished by adding a cross linking agent, which is conventional in the art. An example of a cross linking agent or curative, which is conventionally used with carboxy terminated polybutadiene, is MAPO, which is tris [l-2(-methyl)-aziridinyl]phosphine oxide. It is believed that the trimer acid reacts with the curative exactly in the same manner as the carboxy terminated polymers. The trifunctional character of the trimer acid and low equivalent weight provides a much higher cross link density than can be obtained using just the bifunctional high molecular weight binder material alone.

Though MAPO has been disclosed as a curative, any one suitable for carboxy terminated polymers can be used. Other curatives include polyfunctional epoxy compounds, and other polyfunctional aziridinyl compounds. The amount of curative needed is determined by the amount of binder and trimer acid present. As indicated, the carboxy terminated binders are bifunctional, whereas the trimer acid is trifunctional. When MAPO is used as a curative, for example, there are three functional imine groups present. The amount of MAP0 present should give a total of imine equivalents at least equal to the total carboxyl equivalents in the trimer acid and carboxy terminated binders. Generally, for the composition proportions set forth, a trifunctional curative such as MAPO will comprise from .50 to .85 weight percent of the matrix composition. In the liner compositions, a trifunctional curative such as MAPO will comprise from about 4 to about 9 weight percent of the composition.

In the solid propellant matrix, in addition to the binder, there is present a solid particulate oxidizer such as ammonium perchlorate, ammonium nitrate, and the like. Generally, the oxidizer is present in the amount of to weight percent of the composition. Further, there may be present additional burning rate modifiers such as calcium oxide, milori blue, and the like, in suitable small amounts. Often present are processing casting aids such as Conoco H-25, which is a hydrocarbon type plasticizer made by Continental Oil Co. Additional suitable coating aids include any of the materials listed in copending application Ser. No. 144,265. hereby incorporated by reference, as being suitable for incorporation into the reinforced propellants disclosed therein.

The liner compositions of the invention comprise the binder, which is a carboxy terminated polymer, the trimer acid, a suitable curative, and a carbonaceous filler such as acetylene black in amounts suflicient to provide desired hardness. Generally from 8 to 30 weight percent of the acetylene black is present, with the remainder of the com;- position being the binder. Instead of acetylene black, other conventional materials, such as Philback, carbon black, and the like, can be used.

It is believed the invention will be beter understood from the following detailed examples:

EXAMPLE I Propellant compositions of varying proportions of constituents were formed to determine the effect of the trimer acid. The formulations did not include the aluminum since it was desired to specifically find the effect upon the strength of the basic matrix material from the use of the additive of the invention. A typical mix was made by adding 257.36 grams of carboxy terminated polybutadiene which was Butarez made by Phillips Petroleum Co., to a .7 gallon Baker Perkins mixer. Additionally, added at essentially the same time was 25.74 grams of Emery trimer acid. The oxidizer was a trimodal blend of three different size particles, which is often found in solid propellant mixtures. Thus, also added was one-half of a mixture of 1,150 grams of 400 micron ammonium perchlorate, 500 grams of 80 micron ammonium perchlorate and 500 grams of 10 micron ammonium perchlorate. After half of that mixture of ammonium perchlorate was added, the mixture was then started and mixing continued for three minutes. At the end of this time, the mixing was stopped and the other half of the oxidizer was added. The mixing then continued for 28 minutes under vacuum. The temperature of mixing varied from 160 F. to 170 F. At the completion of the 28 additional minutes of mixing, the mixture was then again stopped and 15.44 grams of MAP curative was added. The mixing then continued under vacuum for an additional minutes. At the completion of mixing, the material was then poured from the mixer into a casting vessel. The casting vessel may have any desired shape. Samples may be cured in 1.5 x 8 x 8 inch pans, and then cut into ASTM dog bone specimens for testing on an Instron tester to obtain mechanical properties. The propellant may be also cast into motor cases and six inch Rohm and Haas test motors and cured for ballistic evaluation. The propellant can be stored at --40 F. to 70 F. for future use in wrapping reinforced grains. The vessel was then placed in an oven maintained at 170 F. for 48 hours, at which time the cure was completely effected.

The same propellant can be prepared and was so done for several examples adding Conoco H-25 as a processing aid. The Conoco H-25 is normally added together with the Butarez and trimer acid during the initial part of the process. In the following Table I, there are two formulations, one without the trimer acid present, the other with the trimer acid. Additionally, it will be noted that almost twice as much MAPO curative is necessitated in the formulation utilizing the Emery trimer acid. This is, of course, necessary in order to effect a sufiicient cross linking of the three carboxy terminated chains of the trimer acid, together with the carboxy terminated polybutadiene.

As can be seen from the above table, the trimer acid serves to significantly increase the tensile strength of the propellant and more than double the modulus thereof. The elongation is decreased. However, this is of no great moment, particularly in propellants having wire reinforcement where the elongation is not as important. The great increase of tensile strength and particularly in the modulus due to the utilization of the small amount of the Emery trimer acid, only .88 gram, was unexpected. Although the invention has been described with regard to the utilization of the propellant matrix of this invention in combination with reinforced propellants, it is not entirely limited thereto. At the lower end of the range of trimer acid, namely, from .85 to .90, the propellant will increase in strength yet maintain fairly good elongation. At the upper range of .10 to 1.10, for example, the elongation is sufliciently effected so that the matrix is more suitable for only reinforced applications. Thus, though the concept has been particularly developed with regard to reinforced propellants, it has been found that it is also suitable for non-reinforced conventional propellant grains at the lower range of trimer acid additive.

EXAMPLE II The typical liner composition of this invention is formulated by placing 1,938.25 grams of carboxy terminated polybutadiene into a .7 gallon Baker Perkins mixer. Additionally, there was added 183.83 grams of Emery trimer acid and 367.75 grams of acetylene black. The acetylene black was added in three essentially equal increments with three to five minute mixing periods after each addition. After the final addition of the acetylene black, the entire charge was mixed for an additional 30 minutes under vacuum. The mixing occurred at either room temperature or at from F. to'17Q F. The temperature range of 160 F. to F. is beneficial in that it lowers the viscosity of the mix and aids air removal in the final 10 minute mixing period under vacuum. After completion of the 30 additional minutes of mixing, 110.25 grams of MAP0 curative was added and the mixing continued under vacuum for 10 minutes at 170 F. temperature. The charge was then poured from the mixer into an ASTM mold and then placed in an oven maintained at 170 -F. The ASTM mold is a closed type and forms a 0.030 inch sheet of liner from which tensile specimens may be cut. The curing time varied from 24. to 72 hours depending upon the physical properties desired. The longer the cure time, the harder'was the liner rnaterial. The following Table II discloses a series of liners with varying amounts of both trimer acid and acetylene black as compared to the two typical liner compositions without the Emery trimer acid. In the following table, the amounts given for the compositions are based on a ratio of material to the carboxy terminated linear polybutadiene binder. This is normally done in cases where a predominant amount of a mixture is a polymeric binder and the remaining materials are expressed in terms of parts per 100 parts of the resin (phr.).

TABLE II Composition, phr.:

carboxy terminated polybutadiene- 100 100 100 100 100 100 l Shore A hardness was measured from bulk cured material having a thickness oi at least 0.5 inch. i

In the above table it can be seen that the presence of the trimer acid significantly increases the hardness of the liner material. For example, the composition containing 10 phr. acetylene black had'a Shore A hardness ata maximum of 32 whereas in a composition containing 10' phr. of trimer acid, and 11 phr. acetylene black had Shore A hardness of 41, or in the case where 20 phr. of trimer acid were utilized the Shore A hardness went up to 55. Additionally, in compositions containing 30 phr. acetylene black, it can be seen that the presence of a relatively small amount of trimer acid again significantlyincreased the hardness of the liner material. Prior to the herein invention, hardness in liner materials had always been controlled by adjusting or modifying the filler material such as acetylene black. Now it has been found, unexpectedly, that the addition of a small amount of trimer acid, which enters into the polymerization rather than act as a filler, will accomplish the hardness control desired.

The novel liner of this invention is particularly useful in solid propellant motors utilizing fiber glass cases. The liner of the invention develops sufiicient cured hardness to support the glass rovings during the case winding operation. In the uncured state, the liner is sufficiently fluid to be applied easily to the grain surface to produce a uniform sheet without surface irregularities, which would tend to produce weaknesses in the fiber glass grain case. Prior to the herein invention, the combination of cured hardness and excellent flow properties in the uncured state was not obtainable by the usual method of adjustment of the carbon black content.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. The method of improving the mechanical properties of solid propellant compositions and liners for solid propellant rocket motors utilizing carboxy terminated polybutadiene binders comprising:

adding to said binders prior to curing a trimer acid formed by reacting 9,11-linoleic acid with the dimer formed by reacting 9,12-linoleic acid with 9,11-linoleic acid in an amount sufficient to improve the mechanical properties of said propellants and liners,

adding an amount of a suitable curative sufficient to effect a cross-linking of both the binders and trimer acid.

2. In solid propellant compositions and rocket motor liners containing carbox terminated polybutadiene binders the improvement which comprises:

adding to said compositions prior to curing an amount of trimer acid formed by reacting 9,11-linoleic acid with the dimer formed by reacting 9,12-linoleic acid with 9,11-linoleic acid suflicient to improve the mechanical properties thereof.

3. Solid propellant matrix compositions and rocket mo tor liners comprising:

suitable fillers, a carboxy terminated polybutadiene binder sufiicient to bind said fillers, and an amount of trimer acid formed by reacting 9,11- linoleic acid with the dimer formed by reacting 9,12- linoleic acid with 9,11-linoleic acid sufficient to improve the mechanical properties of the propellants and liners. 4. A liner of claim 3 comprising: from 8 to 30 weight percent of a carbonaceous strengthening filler, from 4 to 9 weight percent curative, from 10 to 20 weight percent trimer acid, and remainder being' a carboxy terminatedpolybutadiene polymer binder. 5. The liner of claim 4 wherein said curative is tri[1-2(methyl)-iziridinyl]phosphine oxide. 6. A propellant matrix composition of claim 3 comprismg:

from to weight percent solid particulate oxidizer, from .8 to 1.10 weight percent trimer acid, from 8 to 19.5 weight percent of a carboxy terminated polymeric binder, and an amount of curative sufiicient to crosslink said trimer acid and said binder. 7. The composition of claim 6 wherein said curative is present in the amount of .50 to .85 weight percent.

References Cited UNITED STATES PATENTS 3,087,844 4/1963 Hudson et al 149-19 3,177,190 4/1965 Hsieh 260'-94.2 3,257,248 3/1967 Short et a1 149-19 BENJAMIN R. PADGETT, Primary Examiner US. Cl. X.R. 102-103 

